The present invention relates to a film having high tear and impact strength, stiffness and tensile strength in the longitudinal direction (stretched direction) and a process for producing the film. More particularly the invention provides a film for packaging bags which comprises principally a linear polyethylene, is capable of greater reduction of thickness than possible with the conventional films and is suitable for packaging the relatively heavy articles such as rice, grain, fertilizers, etc.
High-pressure low-density polyethylenes produced by radical polymerization under the high-temperature and high-pressure conditions have been popularly used as the material of bags or sacks for packaging heavy articles. In recent years, however, such high-pressure low-density polyethylenes have been rapidly superseded by linear polyethylenes, especially linear low-density polyethylenes.
The linear low-density polyethylenes with few branches produced from copolymerization of ethylene and .alpha.-olefins have many excellent properties, especially strength properties such as tensile strength, impact strength and stiffness, environmental stress cracking resistance (ESCR), heat resistance, heat-sealing characteristics, etc., in comparison with the high-pressure low-density polyethylenes and are used not only as a material for films for packaging bags but also in various other fields.
The non-stretched films or sheets produced from linear low-density polyethylenes according to a T-die or blown film forming (inflation forming) are incapable of reduction of thickness to an extreme extent due to restrictions on forming operations. Further, such non-stretched films or sheets are poor in strength. To eliminate such problems, it has been proposed to subject such films or sheets to a stretching treatment.
A preferred mode of such treatment is biaxial stretching of the non-stretched films or sheets. This treatment, however, necessarily leads to an increased equipment cost. Also, very strict control of operations is required because of the narrow range of stretching conditions. For these reasons, such biaxial stretching has actually been practiced for production of the films or sheets to be used for certain specific purposes.
The known techniques of longitudinal monoaxial stretching require no high equipment cost and are also easy in operational control, but the involve the problem in anisotropy of the stretched film properties, especially tear strength and surface strength in the longitudinal direction (stretched direction) and were unable to produce the stretched films which could safely be put to practical use.
Generally, the packaging bags made by using linear polyethylenes can be made smaller in thickness and have higher strength than those made by using high-pressure low-density polyethylenes, as for instance disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 61-183132 (1986). But the bags made of. linear polyethylenes lack the strength and tensile strength at break of the body portion for realizing a further reduction of thickness. Japanese Paten Application Laid-Open (KOKAI) No. 62-32134 proposes two-layer laminating of the stretched films to improve tensile strength at break of the body portion. In a bag produced from such films, however, when the bag is dropped with the sealed portion down, the body portion of the bag could be elongated in the circumferential direction and deformed, so that such bag is not suitable for practical use. Also, such packaging bag because of its poor stiffness, had the problem of improper filling due to adhesion of the inner faces thereof or buckling of the body portion at the time of automatic filling.
In view of the above, the present inventors have pursued their studies for eliminating said problems of the prior art and producing a thin (stretched) film with excellent strength properties by using linear polyethylenes as the material and, as a result, found that a film having a specific range of heat shrinkage obtained by molding a specific linear polyethylene into a non-stretched film or sheet and stretching it under the specific conditions has excellent tear strength, impact strength, stiffness and tensile strength even when reduced in thickness, and that use of such film for making a packaging bag can improve the stiffness of the packaging bag an its deformation properties under dropping while maintaining the strength of the body portion. The present invention have been accomplished on the basis of this finding.